Sometimes you might like to add some custom CSS to your site to personalize some elements’ styles. In the customizer, please locate to Additional CSS sub section and add your custom CSS code. Like the example showing in the screenshot below:
This is a default feature of WordPress since WordPress v4.7. If you can’t find Additional CSS sub section in your customizer, then it means your WordPress version is lower than 4.7, please update your WordPress to the latest version.
And after adding custom CSS code, if you don’t see any changes, please clear your site cache and your browser cache, then refresh the pages and check again.